This invention relates to control systems for hydraulic circuit means connecting together a variable displacement hydraulic pump and actuator means driven by the pump, with the operating speed of the actuator means being controlled by the position of a displacement volume varying member of the pump, and, more particularly, to a control system for hydraulic circuit means capable of effecting control of acceleration of the actuator means by restricting the operating speed of the displacement volume varying member of the pump to a level below a predetermined maximum speed.
A hydraulic circuit means has been proposed for connecting a variable displacement pump and an actuator driven by the pump, for example, for civil engineering and architectural machines, such as a hydraulic shovel, hydraulic crane, etc., and for hydraulic excavators for mining coals. In a hydraulic shovel, for example, working elements including a boom, a bucket, an arm, travelling members, swivelling member, etc., are driven by the actuator of the hydraulic circuit means which may be a hydraulic motor or a hydraulic cylinder. In this type of hydraulic circuit, the operating speed of the actuator is determined by the displacement volume of the pump. For example, if the displacement volume of the hydraulic pump is suddenly increased by rapidly actuating a displacement volume varying member or a swash plate of the pump connected to a boom cylinder at initial stages of boom operation, the forces moving the boom cylinder would be suddenly increased and give a shock thereto. When it is desired to decelerate the boom cylinder, a great shock also would be suffered if the displacement volume of the pump is suddenly reduced and the boom cylinder might become uncontrollable. Also, if such shock is given when the hydraulic machine is started, the operator would be jolted and the operating lever might become temporarily uncontrollable, thereby causing a hunting to occur.
To obviate the aforesaid problem, in, for example, Japanese Laid Open Patent Application No. 59006/81, it has been proposed to control the displacement volume varying member or swash plate of the variable displacement hydraulic pump while restricting the operating speed of the swash plate to a level below a predetermined maximum speed.
As aforesaid, in the hydraulic circuit means, the operating speed of the actuator is determined by the displacement volume of the pump. The displacement volume is decided by the position of the displacement volume varying member. In a swash-plate pump, the displacement volume varying member comprises a swash plate. Thus, in the hydraulic circuit means using a swash plate pump, the operating speed of the actuator may vary depending on the position of the swash plate, and acceleration of the actuator can be controlled by varying the operating speed of the swash plate.
In the control system of the prior art described hereinabove, when an operating signal indicative of a value for commanding the position of the swash plate is produced, an increasing rate or a decreasing rate of the position command value is compared with a predetermined maximum speed set beforehand. The result of this is that the position command value is increased or decreased at the predetermined maximum speed when the changing rate of the position command value is higher than the predetermined maximum value and at the changing rate of the position command value itself when the changing rate of the position command value is lower than the predetermined maximum speed, and then supplied as an output to swash plate drive means while being compared with the output of a displacement meter for detecting a position of the swash plate. Thus, the swash plate of the variable displacement hydraulic pump is controlled in such a manner that its position is shifted to a position designated by the command value of the operating signal while its operating speed is restricted to a level below the predetermined maximum speed. By effecting control in this way, it is possible to perform small shock and smooth operation of each of the working elements if the maximum operating speed is set at a value suiting each working element driven by the actuator. This is referred to as actuator acceleration control or swash plate speed control.
The predetermined maximum speed is set at a constant value for each operating member and should be set at a low level for a working element of high inertia, such as a boom and travelling members. Because of this, it is impossible to bring the working element of low predetermined maximum speed to a sudden halt when it is desired to stop it. Thus, for example, when a hydraulic machine is to be suddenly braked during travel, there is a great danger. Also, difficulties would be experienced in stopping the working element accurately in a desired position or suddenly reversing the direction of operation of the working element.
Moreover, the predetermined maximum speed is usually set at a suitable constant value at which the movement of the actuator or working element does not slacken and much shock is not caused as the operating lever is suddenly shifted from a neutral position to the full. Thus, the predetermined maximum speed is a value which is too high when the manipulated variable of the operating lever is small or when it is desired to perform a fine operation in which the operating speed of the hydraulic actuator is so low that a shock, no matter how small, will pose a problem. Thus, if the operating lever is suddenly actuated when such fine operation is performed, a shock will be suffered because the actuator is operated at the predetermined maximum speed. Also, in normal operation, an increase in pressure at actuator startup is influenced by the swash plate speed. Thus, if the aforesaid control process of the prior art is used, a peak pressure will be generated at actuator startup, thereby causing a shock.